1. Field of the Invention
The present invention relates to detecting and displaying irregularities in ferrous pipe and, more particularly, but not the way of limitation, to a method and apparatus that employs digital signal processing techniques to produce extremely accurate representations of the irregularities.
2. Description of the Related Art
Ferrous pipe is used in a variety of applications such as the drill string of a well drilling system. Such pipe must be inspected for defects after formation and periodically during use to determine if service induced flaws exist. A pipe containing a flaw such as a crack or loss of metallic area is susceptible to rupturing which results in unnecessary labor as well as a potentially dangerous situation. Accordingly, various systems have been developed to detect flaws in ferrous pipe so that the pipes may be repaired or discarded before rupturing.
Typical systems induce a magnetic field in a ferrous pipe which is then sensed by a bank of magnetic field sensors such as search coils. A motorized mechanical device propels the magnetic field sensors along the length of the pipe or the sensors remain stationary while the pipe feeds past the sensors. The sensors pick up the changes in the magnetic field caused by flaws and produce signals representative of those changes. An analog processing system inputs the magnetic field signals and filters them to remove noise. The analog processor then prints those signals on a strip chart as a one dimensional graph. Spikes on the strip chart which represent large changes in magnetic flux indicate flaws in the pipe. Accordingly, after the printing of the strip chart, a system operator can try to match the chart to the pipe in an attempt to locate the suspected flaw using any of a variety of prove-up methods.
Although the analog signal processor produces usable information indicating the location of flaws in ferrous pipe, it suffers from several disadvantages. First, analog signal processing limits the amount of information available to the user. Illustratively, an analog signal processor does not provide frequency information because it cannot determine frequency without continuous user monitoring. Such user monitoring would diminish inspection speed to a point where it would be economically unfeasible. Finally, an analog signal processor does not provide discrete values representing flaws which makes locating flaws as well as determining their severity difficult. Consequently, unflawed pipe is often unnecessarily discarded, while flawed pipe might be used.
U.S. Pat. No. 5,030,911 which issued Jul. 9, 1991 to Lam discloses a tester for new pipes that uses digital signal processing in an attempt to improve over analog signal processing. The Lam tester includes search coils that traverse a pipe to detect changes in a magnetic field induced in the pipe. A digital computer digitizes and processes the magnetic field information for display on a two dimensional plot. The processing performed by the computer consists of threshold or signal magnitude evaluation. To perform the threshold evaluation, an operator inputs a threshold value which the computer then compares to the incoming signals. The computer processes any signal with a magnitude above the threshold and does not process any signal below the threshold. The computer prints the stored signals on a two dimensional plot representing the circumference of the pipe to provide an indication of defects. The Lam tester provides a second graph below the first graph to display the digital equivalent of all the analog values input into the computer. Consequently, the second graph consists of the digitized versions of the analog signals with no additional processing. An operator then uses the two graphs in tandem to determine where defects exist along the new pipe tested.
Although the Lam new pipe tester includes a computer for digital signal processing, it does not provide real time processing because the two dimensional plot cannot be generated until the entire length of the pipe has been scanned. Furthermore, the Lam system only performs threshold evaluation and does not perform any other type of processing such as frequency analysis. Without additional processing, the computer fails to properly eliminate noise from the magnetic field signals resulting in false indications of pipe flaws. Thus, it is often time consuming to locate flaws because a complete manual inspection of the pipe must be performed to distinguish real flaws from false indications. Consequently, the Lam tester is limited to the detection of flaws in new pipes and is unable to analyze used pipe.
Accordingly, a method and apparatus that performs digital signal processing and analysis in conjunction with improved magnetic field detection to precisely indicate and quickly locate flaws would significantly improve over the above described systems because it makes the inspection of both new and used ferrous pipe possible while reducing defect location time to a minimum.